1. Field of Invention
The present invention relates to the field of optical elements, and more particularly to the packaging of optical elements. The invention is yet further related to packaging of retarder type optical elements.
2. Discussion of Background
A retarder is an optical element often used in optical systems. Retarders can be made from a variety of materials. One important class of these materials is plastics such as polycarbonate. A design that includes a retarder may require that it be optically bonded to other components within the assembly. These “other” components are often various types of glass. Accomplishing the bond can present a challenge in that any adhesive used for this purpose should meet a number of requirements. The following is a list of some of these requirements:
Be optically clear.
Adhere to the plastic, glass, or any thin film coating that may be present in a device or other optical configuration to which the retarder is to be affixed.
Not chemically react with the plastic, glass or any thin film coating that may be present.
Not yellow or otherwise “age” with exposure to intense light flux.
Have a coefficient of thermal expansion similar to that of the plastic, glass, or any thin film in the device or optical configuration to which it is affixed.
Have an index of refraction close to that of the plastic, glass, or any thin film in the device or optical configuration to which it is affixed.
Have a suitably long shelf life.
Have a suitably long pot life (pot life=glue stays liquid or gel-like until cured), so that the optical elements can be placed in their required positions prior to the adhesive fully curing.
Be simple to prepare (such as mix and degas for two part adhesives).
Have a viscosity appropriate to the means of application (thick enough to work with, but thin enough so that it adequately and evenly coats all areas to the glued).
Be inexpensive to buy and use.
Ideally, the adhesive should meet all of these requirements. The reality is that, for some applications, such an adhesive may not exist, particularly in high light or heat intensive environments. One specific application for which these requirements are difficult, or currently impossible to meet is in a prism assembly of a video projector. The result is a practical problem: how can the retarder be efficiently included in the prism assembly of a video projector?
A partial solution to this problem (adopted by at least one company that sells wavelength specific retarder materials to the prism industry) is to produce a retarder “sandwich”. An example is illustrated in FIG. 1, which includes a retarder 100, cover glasses 110, and 120, and rigid adhesive 130. As shown, the retarder 100 is laminated between the two cover glasses 110 and 120. The purpose of creating such a product is to make it easier for the customer to laminate the retarder into their optical assembly. The approach is successful in that customers need deal with only the relatively simpler task of laminating the outer glass surfaces of the sandwich to other glass components (e.g., prism(s)) in their optical assembly. On the other hand, the fabricator of the sandwich is still left with the more difficult task of laminating the retarder to the cover glasses, which generally includes the difficulties and requirements listed above, and at elevated cost.
Consider the details of the sandwich illustrated in the FIG. 1. The sandwich is composed of a retarder 100 that has been laminated between cover, glasses using a rigid adhesive 130. This is representative of retarder sandwiches commercially available at this time. The requirements on the bonding adhesive that are listed above must still be addressed in fabricating the sandwich. The result is a sandwich that is expensive and difficult to manufacture. More importantly, the result is a sandwich that may still not fully meet the requirements.
Stress build up and the negative effects of stress (e.g., stress induced birefringence) is a problem also found in optical devices other than retarder sandwiches
Most modern optical communication systems utilize photonic pathways and Optical-Electronic-Optical (O-E-O) control elements. (In this document, the term control elements refers to components that perform functions such as multiplexing, demultiplexing, routing, etc.). Although capable of performing the required tasks, O-E-O control elements are expensive, “slow” to respond, incapable of handling signals not conforming to standard data rates and consume significant power. One approach to addressing the deficiencies of such control elements is through the use of Optical-Optical-Optical (O-O-O) control elements. During the past few years, many different types of O-O-O control elements have been proposed. The functioning of some of these is based on the manipulation of polarized light. A small sample of such devices can be found in the following U.S. Pat. Nos. 4,679,894; 4,711,529; 4,720,171; 4,720,172; 4,720,174; 4,737,019; 4,749,258; 4,755,038; 4,773,736; 4,781,426; 4,784,470; 4,790,633; 4,792,212; 4,813,769; and 4,913,509.
It is important to the proper operation of any optical device that manipulates polarized light that the polarization not be altered by spurious and/or uncontrolled optical effects. More specifically, it is important that stress induced birefringence be minimized along the optical path. Origins of stress can include that stress built into the optical assembly during its fabrication and that stress generated by change in temperature (due to differing Coefficients of Thermal Expansion (CTE) between the various components within the optical assembly). Note that one feature common to the configuration of almost all previously disclosed O-O-O control elements is that the components are held together using an adhesive. The significance of this point is that the rigid bond between the optical components is not “accommodative”. That is, the rigid bond transmits stress and allows the stress to build up rather than providing a mechanism for stress reduction.